1. Field of the Invention
The present invention relates to optical spectrometers and in particular to a new class of flexible fiber optic or integrated optical spectrum analyzers for UV, VIS, NIR and MIR ranges of spectrum which do not require dispersion gratings, expensive mechanical parts or additional optical sources. The spectrum analyzer can be used with or in fiber optic chemical and biological sensors to monitor samples in situ instead of testing them in the laboratory.
2. Information Disclosure Statement
Optical spectrum analyzers have many important applications for monitoring the spectra of optical signals. They can be used, for example, in various fiber optic chemical and biological sensor systems designed to detect concentrations of different species. Each specific species to be detected has a certain characteristic absorption line, a so called fingerprint, which can be selected with the help of spectrum analyzers. Intensity of this selected spectrum line is proportional to the concentration of species to be detected.
There are many spectrometers available on the market. Most of them use precision diffraction gratings and expensive precision mechanics to achieve their performance. These spectrometers are rather large and expensive devices which can be used for detection of a signal from optical fiber sensors in laboratory, but not in field conditions. Moreover, for operation of most fiber optic sensors it is not necessary to have complete spectrum of the radiation. The spectrum analyzer could be tuned to one or two specific lines of spectrum, i.e. to the fingerprint of the specific species to be detected. For this and other applications it is desirable to have a compact inexpensive spectrum analyzer which can be easily adjusted to each specific spectrum line.
A miniature fiber optic spectrometer S-1000 has been recently developed by Ocean Optics, Inc. for operation in UV, VIS and NIR ranges of spectrum This spectrometer, however, is not able to operate in Middle Infrared (MIR) range of spectrum which is the most important region for chemical sensors operation, since the most useful fingerprints of many materials are located just in this range of the spectrum
In a recent paper by D. D. Marcenac and J. E. Carroll ("Maximum-entropy optical spectrum analyzer" Optics Letters 20, (1995) 1074-1076) a new type of spectrometer is demonstrated which has no movable parts, no additional optical source and no precision engineering. The spectrometer operates by transmitting the laser fight through two single-mode fibers to generate an interference pattern in the far field whose brightness is shown to be a measure of a optical signal's autocorrelation. The interference pattern is captured with an infrared camera using a charge coupled device (CCD) and is transferred to a computer. The computer is used for numerical processing of information in order to extract the spectrum of the light form the interference pattern. This spectrum analyzer, however, can not operate on real time scale as is desirable for many sensing applications. It also can not operate with multimode waveguides generating a complicated speckle pattern (modal noise) which disturbs the signal interference pattern. As a result, this spectrum analyzer can not be used in the middle infrared range of spectrum where single-mode fibers are still not available.